Take it you are talking Mountain bike? Sintered pads or standard? Muddy conditions or Bone dry? Steep hills or flat ground? Too many options for a set time.

I ride a Tandem, use sintered pads, and ride a lot in winter. Check my pads once a year and they normally last for 2 years- or about 4 to 5,000 miles. Then there is the make of brake- original or copy pads--- Just keep taking them out and see how they are lasting.

If they go 2000-5000 miles as you guys have stated, then thats good for me. I was expecting something like 300 miles or so. Ive never had disc before so I didnt know. Either way thats longer than I expected.